Digital lighting technologies, i.e., illumination based on semiconductor light sources, such as light-emitting diodes (LEDs), offer a viable alternative to traditional fluorescent, HID, and incandescent lamps. Functional advantages and benefits of LEDs include high energy conversion and optical efficiency, durability, lower operating costs, and many others. Recent advances in LED technology have provided efficient and robust full-spectrum lighting sources that enable a variety of lighting effects in many applications. Some of the fixtures embodying these sources feature a lighting module, including one or more LEDs capable of producing different colors, e.g. red, green, and blue (RGB), as well as a processor for independently controlling the output of the LEDs in order to generate a variety of colors and color-changing lighting effects, for example, as discussed in detail in U.S. Pat. Nos. 6,016,038 and 6,211,626, incorporated herein by reference.
White light can be produced by mixing different colors of light generated using multiple LEDs. There are several techniques for characterizing white light. In one technique, color temperature is used as a measure of the color of light within a range of light having white characteristics. A correlated color temperature (CCT) of the light represents the temperature in degrees Kelvin (K) of a black body radiator which radiates the same color light as the light being characterized.
Another technique for characterizing white light relates to the quality of the light. In 1965 the Commission Internationale de l'Eclairage (CIE) recommended a method for measuring the color rendering properties of light sources based on a test color sample method. This method has been updated and is described in the CIE 13.3-1995 technical report “Method of Measuring and Specifying Colour Rendering Properties of Light Sources.” In essence, this method involves the spectroradiometric measurement of the light source under test. This data is multiplied by the reflectance spectrums of eight color samples. The resulting spectrums are converted to tristimulus values based on the CIE 1931 standard observer. The shift of these values with respect to a reference light are determined for the uniform color space (UCS) recommended in 1960 by the CIE. The average of the eight color shifts is calculated to generate the General Color Rendering Index, known as CRI. Within these calculations the CRI is scaled so that a perfect score equals 100, where perfect would be using a source spectrally equal to the reference source (often sunlight or full spectrum white light). For example, a tungsten-halogen source compared to full spectrum white light might have a CRI of 99 while a warm white fluorescent lamp would have a CRI of 50. Artificial lighting generally uses the standard CRI to determine the quality of white light. If a light yields a high CRI compared to full spectrum white light, then it is considered to generate better-quality white light.
The CCT and CRI of light can affect the way in which an observer perceives colors in the observer's environment. An observer will perceive the same environment differently when viewed under lights producing different correlated color temperatures. For example, an environment that looks normal when viewed in early morning sunlight will look bluish and washed out when viewed under overcast midday skies. Further, white light with a poor CRI may cause colored surfaces to appear distorted or unappealing to the observer.
Due to the differences in perception of an environment under different lighting conditions, the color temperature and/or CRI of light may be critical to creators or curators of particular environments. Examples include architects for buildings, artists for galleries, stage directors for theaters, etc. Additionally, the color temperature of artificial light affects how observers perceive a display, such as a retail or marketing display, by altering the perceived color of items such as fruits and vegetables, clothing, furniture, automobiles, and other products containing visual elements that can greatly affect how people view and react to such displays. One example is a tenet of theatrical lighting design that strong green light on the human body (even if the overall lighting effect is white light) tends to make the human look unnatural, creepy, and often a little disgusting. Thus, variations in the color temperature of lighting can affect how appealing or attractive such a display may be to observers.
Moreover, the ability to preview a decoratively colored item, such as fabric-covered furniture, clothing, paint, wallpaper, curtains, etc., in a lighting environment or at a color temperature that matches or closely approximates the conditions under which the item will ultimately be viewed by others would permit such items to be more accurately matched and coordinated. Typically, the lighting used in a display setting, such as a showroom, cannot be varied and is often chosen to highlight a particular facet of the color of the item, leaving a purchaser to guess as to whether the item in question will retain an attractive appearance under the lighting conditions where the item will eventually be placed. Differences in lighting can also leave a customer wondering whether the color of the item will clash with other items that cannot conveniently be viewed under identical lighting conditions or otherwise directly compared.
Some multichannel LED fixtures that produce white light allow a user to control the color temperature of light generated by the LED fixture by adjusting the brightness of each individual LED in the LED fixture. To adjust the characteristics of the white light, the LED fixture must have the capability of recreating various correlated color temperatures. Typically, this has been accomplished by using multiple white LEDs having different CCTs, or by combining multiple color LEDs, such as red, green, and blue to generate a desired white color. However, LED fixtures that use prime colors, such as red, green and blue, produce saturated light that cannot generate all colors in the gamut. Such fixtures also do not allow high granularity of control due to the large size of the gamut. In addition, a fixture with multiple discrete white LEDs having different CCTs will have a very small gamut along the black body. As a result, the fixture will not be able to generate all white color points on the black body locus.
Moreover, it is known that the human eye does not perceive “true” white light as white points on the black body locus. Rather, the human eye perceives “true” white light as white points above and below the black body locus, depending on the CCT of the light. Conventional discrete white LED fixtures are unable to compensate for individual color perception (hue) along the CCT isothermal lines above and below the black body locus because they cannot produce light at the “true” white color points. Thus, conventional white LED fixtures do not correct for perception of “true” white by the human eye.
As discussed above, a high CRI equates to a high quality of light. Conventional multichannel LED fixtures are incapable of generating high CRI values across a broad range of color temperatures, e.g., between approximately 2700° K and 6500° K. For example, conventional white LED fixtures can only generate CRI values of 82 or less across this range of color temperatures. Conventional RGB fixtures perform even worse, with CRI values no greater than 33 across a similar range of color temperatures.
A conventional RGB LED fixture may encompass the entire black body, but due to the limitations in efficiency of the individual LEDs used to generate the light at various points along the black body, the overall efficiency of the system is poor. For example, the efficiency of one conventional RGB LED fixture is approximately 40-42 lumens/watt across the above-mentioned range of color temperatures. A conventional white LED fixture achieves between 38 and 56 lumens/watt across the same range of color temperatures. There are existing fixtures that utilize a combination of red-shifted white LEDs and green-shifted white LEDs to generate white light at higher efficiencies than white LEDs of the same color temperature. However, this combination doesn't allow the color and hue to be tuned as discussed above to correct for perception of “true” white.
Another important consideration for adjustable illumination sources is the lumen output across the gamut, which relates to the efficiency as well as the quality of the light produced. However, conventional white LED and RGB LED fixtures may produce less than 350 lumens over an approximately 2700° K to 6500° K range of color temperatures.
Thus, there is a need in the art to provide a multichannel white light source of illumination capable of true generation of all white color points on or near the back body locus within the gamut that can be optimized for high CRI across a broad range of color temperatures and provide greater overall system efficiency and light output, and that may optionally overcome one or more drawbacks with existing solutions.